This invention relates to the general area of wastewater processing, and in particular to the use of low concentrations of ozone in oxygen to reduce the biochemical oxygen demand of the wastewater prior to further waste treatment.
In any body of water a small concentration of oxygen is dissolved in the water. The dissolved oxygen (DO) is considered to be one of the most important assets in good water quality for streams, rivers and lakes. The importance of dissolved oxygen in natural bodies of water is based upon the need for fish and other aquatic organisms to breath oxygen. The presence of organic waste in streams, rivers and lakes provides an alternative use or competitive situation for use of the dissolved oxygen.
This competition for oxygen arises from the need for oxygen to be used by bacteria in attacking the organic waste contained in the water. When bacteria metabolize the organic waste found in the lake or stream, the bacteria utilize some of the dissolved oxygen in the body of water to break down the organic materials into simpler compounds such as CO.sub.2 and H.sub.2 O. The bacteria rely on the energy released in such breakdown reactions for their own growth and reproduction.
As the amount of organic waste in a body of water increases the activity of bacteria in the body of water also increases, and the bacterial demand for oxygen increases. This increased consumption of the available dissolved oxygen by the bacteria in metabolizing dissolved organic wastes reduces the dissolved oxygen in the water which is available for fish and other aquatic organisms. In extreme cases bacterial activity in a polluted body of water becomes so great that the dissolved oxygen severely depletes and suffocation of the aquatic organisms occurs.
The tenuous nature of this situation is quickly apparent upon understanding that oxygen is only slightly soluble in water and at 20.degree. C. oxygen is present in water at about only 9 mg/L or 9 ppm. Thus it will be appreciated that due to the slight solubility of oxygen in water, the competition among organisms deriving their oxygen supply from the water is acute.
In the past 30 years increased pollution of streams and water supplies by industrial waste, has made it necessary to monitor streams, rivers and lakes by assessing the amount of dissolved oxygen in the body of water and by determining the consumption requirements for that oxygen by the inhabitants of the water supply. This measuring of oxygen consumption is called the biochemical oxygen demand (BOD) and measures the utilization of available dissolved oxygen in biochemical reactions occurring within a water supply.
As a practical matter, the consumption of oxygen by fish and other large aquatic creatures is not measured by biological oxygen demand. Rather, it is the demand for oxygen exerted by the chemical reactions and the bacteria metabolizing organic materials in the water supply which is determined through BOD measurements. Therefore, biochemical oxygen demand is relied upon to indicate the amount of pollution present in the stream and to indicate the status of the water supply as a life-support system for fish and other aquatic life.
By comparison, a very clean body of water will have a biochemical oxygen demand over a period of five days of about 1 mg/L. This consumption of oxygen in a clean body of water is attributable to bacterial action on the naturally occurring organic materials from decaying leaves and animal wastes. When BOD values for a five-day period, or the "BOD.sub.5 " value, exceeds 10 mg/L, it is an indication of the presence of sewage pollution in the body of water.
Because it is critical to maintain minimum levels of dissolved oxygen in natural bodies of water and to limit the competition for oxygen so aquatic life is not suffocated, industrial processes which release water into streams and rivers and into water-treatment facilities must be monitored for organic pollution levels. The monitoring of organic pollutants in effluents must be conducted so organic waste in the discharge water does not raise the BOD of the receiving stream to levels which compromise aquatic life.
In gauging the suitability of water for release from an industrial process such as a meat-packing house or food-processing waste from organic processes, it has become the method of choice to measure the biochemical oxygen demand of the process discharge water. When necessary the discharge water can then receive treatment to reduce the organic wastes which will reduce the BOD of the receiving water stream.
Once reduction is achieved in the organic waste which will cause BOD elevation the water can then be safely discharged from the industrial facility and into the receiving stream. In this manner a safe level of dissolved oxygen can be maintained in the receiving stream and harm to the aquatic environment and the indigenous fish and other life existing in the river or stream can be preserved. In the situation in which the industrial water is being discharged into a municipal water treatment facility, a dissolved oxygen level can be maintained which will not harm the bacteria culture present in a water treatment facility.
In practice, industrial organic water waste producers are identified by local water authorities and are provided with a biochemical oxygen demand goal which it must attain for release of the process water into the public waterway. Failure to meet the BOD goals results in fines against the industry by the local water protection authority. Therefore, industries producing wastewater are confronted with finite and specific goals to achieve before wastewater can be released from the plant.
The reasons for failing to achieve the assigned BOD requirements can range from equipment failures during the water processing to the industrial plant concluding that it is less expensive to pay the fine than to treat the wastewater. It may also be the case that the process water is released prematurely and without attaining the BOD goal as the plant is without the means to store wastewater requiring additional treatment.
Therefore it is an object of the present invention to provide a method of reducing the biochemical oxygen demand and chemical oxygen demand and ammonia and total nitrogen of wastewater which is inexpensive to use and which allows the reduction in existing treatment process costs.
It is another object of the present invention to provide an efficient method of reducing biochemical oxygen demand and chemical oxygen demand and ammonia and total nitrogen which can be added onto existing water treatment facilities to supplement the existing treatment process.
It is a primary object of the present invention to provide a method of reducing biochemical oxygen demand through the injection of a mixture of ozone and oxygen gases into the wastewater and where the ozone concentration is sufficiently low so as to avoid the high costs associated with the generation of large quantities of ozone.